(a) Field of the Invention
The present invention relates to a method and an apparatus for transmitting a signal using an unlicensed band in a cellular system.
(b) Description of the Related Art
In accordance with an increase in users of mobile Internet using a mobile communication system, methods for increasing capacity of the mobile communication system have been sought. Among these methods, one method is to increase a bandwidth for the mobile communication system, which may be achieved by additionally securing a frequency of a licensed band. However, it costs a lot to license and use the frequency of the licensed band, and the frequency of the licensed band allocated for the mobile communication system is limited. Accordingly, a method for providing the mobile communication system using a frequency of an unlicensed band or a TV white space in which a lot of relatively available frequency bands exist and costs are also inexpensive has been considered. Further, a method for sharing the frequencies of the licensed/unlicensed band between operators of the mobile communication system has also been considered.
A communication system using a frequency of a band in which the frequency is shared (hereinafter, referred to as “unlicensed band”) has the following limits.
First, in order to minimize an influence on another system sharing the frequency of the unlicensed band, transmission power is limited. Therefore, in the case in which a licensed band system and an unlicensed band system are installed on the same place, a coverage hole may occur.
In addition, for a fair coexisting with a neighboring unlicensed band system, the frequency of the unlicensed band should be discontinuously or randomly used. As a result, transmission reliability of a control channel, a common channel, and the like of the mobile communication system may be decreased.
Further, a regulation on the frequency of the unlicensed band should be obeyed. A system using the frequency of the unlicensed band should perform a clear channel assessment (CCA) for data transmission, and determine whether or not the channel is used depending on the result of CCA. A device (or system) occupying the channel according to the result of CCA may have a channel occupancy time limited according to a frequency regulation, may not occupy the channel during a time exceeding a maximum channel occupancy time, and needs to additionally perform the CCA in order to re-occupy the channel.
Due to the limit of the unlicensed band system described above, rather than a standalone system that independently uses the unlicensed band, a scenario in which the unlicensed band system is installed/operated complementarily with the licensed band system has been examined. In this scenario, control functions that require reliability such as a terminal control, a mobility management, and the like are performed by a system operated in the frequency of the licensed band, and traffic functions such as an increase in wireless transmission speed, a wireless traffic load distribution, and the like are operated in a form complemented by the unlicensed band system. That is, carriers of the licensed band perform the control and traffic functions, and carriers of the unlicensed band perform the traffic function. The above-mentioned operation may be implemented by a carrier aggregation (CA) technology, and as an example of the CA, an LTE carrier of the licensed band, an FDD carrier of the unlicensed band, or a TDD carrier of the unlicensed band in which an uplink and a downlink are simultaneously operated may be respectively aggregated.
The cellular system may utilize an advanced interference control technology, and inexpensive and rich frequency resources of the unlicensed band to thereby guarantee service quality at the time of providing a mobile communication service. In this case, in order to secure the above-mentioned advantages under coexistence of a variety of regulations of the unlicensed band and other unlicensed band systems, new coexistence technology and interference control technology are required. Particularly, the carrier aggregation technology that considers characteristics of the licensed band and the unlicensed band, and an operation accordingly are required. In addition, by providing reliability of wireless transmission, and reflecting limits (characteristics) for operating the device of the unlicensed band, a service equivalent to an existing cellular system should be provided.